Lubricating compositions comprising esters of tricarboxy acids



United States Patent M 3,102,098 LUBRICATING COMPOSITIONS COMPRISING ESTERS OF TRICARBOXY ACllDS Stephen J. Metro, Scotch Plains, and Alfred H. Matuszak, Westfield, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed May 1, 1959, Ser. No. 810,249 9 Claims. (Cl. 252-56) This invention relates to esters of aliphatic tricarboxy acids and to lubricating oil compositions containing said esters. Particularly, the invention relates to synthetic esters useful as lubricating oils and oil additives, said esters being prepared from monohydric alcohols with aliphatic tricarboxy acids, wherein said acids have a total of 6 to 7 carbon atoms.

The use of various aliphatic diesters and complex synthetic esters as lubricating oils is well known to the art and have been described in numerous patents, eg US. 2,723,286; 2,743,234; and 2,575,196. In general, these prior aliphatic synthetic ester lubricating oils are prepared from dicarboxylic acids and have viscosity properties that are outstanding at both low and high temperatures, especially when compared to mineral oils. Because of these characteristics, synthetic ester oils have become of incerasing importance in the field of lubrication and one of the most important current applications of such compounds is in the lubrication of aviation gas-turbine systems such as are used in the turbo-jet and turboprop type of aircraft. However, in general, the loadcarrying ability of the aliphatic ester oils prepared from dicarboxylic acids is not particularly high. Because of the increasing severity of the conditions prevailing in the lubrication of aviation gas-turbine sytsems, it is highly desirable to form synthetic ester lubricating compositions having higher load-carrying ability than is now generally available and yet at the same time being non-corrosive. It has now been found that partial esters of certain tri carboxy acids are useful as load-carrying agents in lubricating oils, particularly diester and complex ester lubricating oils. While many prior load-carrying agents greatly increase lead corrosion, these partial esters of tricarboxy acids do not corrode lead. This is particularly important because of the lead-containing bearings frequently used in aircraft engines. The full esters, i.e. the triesters of the tricarboxy acids of the invention, are useful per se as synthetic lubricants and have many of the same properties which have made the aliphatic diesters, e.g. di(2- ethylhexyl) sebacate, outstanding lubricants. Thus, generally speaking, the triesters of the invention, particularly when branched, have low pour points, good viscositytemperature relationships, high flash points and are un usually thermally stable. These triesters are superior to the diesters used in preparing commercial synthetic aircraft lubricants in that they have outstanding stability towards viscosity increase and oxidation when operating at high temperatures.

The esters of the invention include mono-, di-, and triesters of C to C alkanetrioic acids with C to C alk-anols. The preferred acids are 1,2,3-tricarboxy propane (tricarballylic acid) and 1,2,4-tricarboxy butane.

Preferred alcohols for forming the ester are those alkanols of about 3 to about 20, preferably to 13, carbon atoms. These alcohols may be either straight chain or branched chain alcohols. Primary alcohols are particularly preferred. Secondary and teritary alcohols, while operable, are less preferred for the purposes of this invention, since esters prepared from such alcohol generally have poorer thermal stability than the corresponding esters prepared from primary alcohols. Among the straight chain primary alcohols operable in preparing the Patented Aug. 27, 1963 synthetic lubricants of this invention may be mentioned n-decyl alcohol, n-octyl alcohol, n-heptyl alcohol, n-hexyl alcohol, n-amyl alcohol, n-butyl alcohol, and Fischer- Tropsch synthesis alcohols. Operable branched chain primary alcohols include 2-ethylhexyl alcohol, Z-ethylbutyl alcohol, and the Oxo alcohol. The Oxo alcohols are Well known in the art. They are prepared in a two stage reaction. The first stage involves reacting olefins, such as polymers and oopolymers of C and C monoolefins, with carbon monoxide and hydrogen at temperatures about 300 to 400 F. and pressures of about 30 to 400* atmospheres, in the presence of a suitable catalyst, ordinarily a heavy metal carbonyl, such as cobalt carbonyl, to form a mixture of aldehydes having one carbon atom more than the olefin. In the second stage, the aldehyde mixture is hydrogenated, to form an isomeric mixture of highly branched chain primary alcohols which is recovered by distillation. The process is well known and has been described in various US. patents, e.g. US. 2,327,066 and US. 2,593,428.

The tricarboxy acid esters of the invention are repaired by conventional esterification techniques. The esterification is carried out by reacting l to 3 molar proportions of an alcohol (depending upon whether a full or partial ester is desired) per one molar proportion of the tricarboxy acid, under reflux conditions. Generally, a waterentraining agent, eg. heptane, toluene, etc., is used, and the reaction is carried out until the calculated amount of water is removed overhead. When the full ester is desired, a slight excess of alcohol can be used in order to insure completion of the reaction. Esterification catalysts, e.g. sodium bisulfate, sulfuric acid, toluene sulfonic acid, soduim methylate, calcium oxide, etc., are generally used when the complete esters are made, although the reaction may be carried out Without a catalyst. When partial esters are made, no catalyst is generally employed.

This eliminates the necessity for filtering and washing to remove the cataylst which otherwise might tend to make the product corrosive. In any case, after the desired amount of water is removed, the remaining reaction product may be filtered and Washed if a cataylst was used and then distilled under vacuum in order to remove the entraining agent and any unreacted alcohol overhead.

Various esterification methods involving polycarboxylic acids have been described in several patents, eg US. 1,993,73 6, US. 2,249,768, US. 2,766,273 and US. 2,015,- 088, which methods are generally applicable to the materials of the present invention.

The resulting ester product may be used as a lubricant per se or it can be advantageously blended in any proportions with other lubricating oils. The full esters of the higher molecular Weight alcohols, particularly the 0x0 alcohols, seem to have the best all aroundproperties for use as a lubricant per se, While the partial esters are best used by blending with other oils. Thus, such blends may contain for example, about 0.25 to 90, preferably 0.25 to 70, Weight percent of the tricarboxy acid ester and about 99.5 to 10, preferably 99.5 to 30 Weight percent of another lulbricating oil. The lubricating oil used with the trioarboxy acid esters may be a mineral lubricating oil, a synthetic lubricating oil or any mixtures thereof. Particularly preferred synthetic oils for blending with the trioarboxy acid ester materials are the saturated aliphatic diesters represented by the formula:

wherein R is a straight or branched chain hydrocarbon radical of a C to C allsanedioic acid, R represents an alkyl radical of a C to C branched or straight chain alkanol and the total number of carbon atoms in the molecule is about 20 or more. Specific examples of such diesters include di(2-ethylhexyl) sebacate, di(C Oxo) pelargonate, di(C Oxo) adipate, etc. Other synthetic oils which may be used will include esters of monobasic acids (e.g. C Oxo alcohol ester of C Oxo acid), esters of glycols (e.g. C Oxo acid diester of tetraethylene glycol), complex esters, esters of phosphoric acid, halocarbon oils, sulfite esters, silicone oils, carbonates, formals, polyglycoltype synthetic oils, e.g. from trimethylol propane, trimethylol ethane, neopentyl glycol, and pentaerythrytol.

Various other additives may also be added to the lubrieating compositions of the invention in amounts of about 0.1 to 10.0 weight percent, based on the total weight of the composition. For example, rust preventives such as calcium petroleum sulfonate; V.I. irnprovers such as the polymethacrylates; oxidation inhibitors such as phenylalpha-naphthylamine, para-amino diphenyl-amine or phenothiazine; corrosion inhibitors such as sorbitan monooleate; pour point depressants; dyes; grease thickeners; and the like may be added.

The invention will be further understood by the following examples:

EXAMPLE I A tri C x0 ester of 1,2,4-but-ane tricarboxy acid CH (COOH)OH(COOH)CH CH COOH was prepared as cfollows:

A. Into a 1,000 ml. round bottom three-necked flask equipped with a stirrer, thermometer, reflux condenser and water tra was placed 429 grams (3.3 moles) of C Oxo alcohol (prepared by subjecting butylene-propylene copolymer to the 0x0 process) and 190 grams (1 mole) of 1,2,4-tricarboxy butane acid. The molar excess of alcohol was used in order to insure complete esterification. 1.5 grams of sodium bi-sulfiate as a catalyst and 150 ml. of heptane as a water entraining agent were next introduced. The mixture was then refluxed at atmospheric pressure and stirred vigorously for 2 /2 hours. During this time the calculated amount of water (3 moles) collected in the trap. The resulting oil residue was stripped of volatiles at 200 C. under a jet of nitrogen. The residue was filtered free of catalyst.

B. A tri C Oxo ester of 1,2,4-tricarboxy butane acid was prepared by following the procedure of A above, but using the C Oxo alcohol (prepared by subjecting tritricarballylic acid prepared by standard esterification techniques were submitted to the following tests:

Lead Corrosion T est 0.5 wt. percent of phenothiazine, as an oxidation inhibitor, was added to the ester oil to be tested. The resulting composition was then tested by rapidly rotating a bi-metallic strip consisting of a lead strip and a copper strip bound together in the oil sample maintained at 325 C. while air was bubbled through the sample. The weight loss of the lead strip was then determined, and reported in terms of milligram weight loss per square inch of lead surface (mg/m Measurements were made at different intervals of time. Lubricants giving very low lead corrosion are desired for aircraft engine lubrication. Low lead corrosion at long intervals of time, say 12 hours, is an indication or good storage stability.

Oxidation-Corrosion Stability (O.C.S.) T est The ester compositions containing the 0.5 wt. percent of phenothiazine were also tested for corrosiveness to copper, magnesium, iron, aluminum and silver and for change in viscosity and acidity. Several of these tests were carried out in accordance with MIL-L-7808C specification procedure, i.e., by immersing weighed strips of the metal to be tested in 100 cc. of the sample maintained at 347 F. for 72 hours while bubbling 0.5 liter per hour of air through the sample. Other oxidation tests were carried out in the same manner but at 500 F. for 24 hrs. The metal strip is then reweighed to determine the weight change as mg./cm. and the viscosity ('cs. at 100 F.) and neutralization number (mg. KOH/ gm.) of the oxidized composition is determined.

Ryder-Gear Load Carrying T est The load carrying ability of the synthetic ester was determined by Ryder gear test in accordance with the MlLL-7808C specification procedures.

In addition to the above tests, the usual determination of viscosities, flash point, fire point, and other standard measurements were made.

The composition tested and the results obtained are summarized in Table I which follows:

TABLE I 1,2,4,-Tricarb0xy Tricarballylic Acid Tricstcrs Butane Triesters Properties CaOXO CwOxo Butyl 2-Ethy1- CgOXO CwOxo C Oxo hexyl Viscosity:

at 210 F. (05.).. 4.19 5. 41 2. 22 4.23 4.60 10. 52 10.6 at 100 F. (as). 21. 39 32. 25 8. 56 25.05 27. 27 109. 4 111. 4 at 40 F. (cs.) 12,042 35, 478 22, 983 30, 295 Flash, (OOC) F 450 430 375 435 485 525 520 Fire Point, F 515 540 420 500 520 610 590 Actual Pour Point, F 65 75 65 35 25 Lead Corrosion Test:

1 Hr., mg./m. 0.02 0. 2 4 Hr., mgJin 2 0.04 8 Hr., mg./in. 0.7 12 Hrs. Big/in. 3. 0 3. 0 Total Acid N0., mgs. KOH/gm 0.22 0.03 0.1 Oxidation Corrosion Stability 1 (mg/sq. cm. change):

Cu 0. 14 16 0. 04 0. 23 Mg 24.96 0.03 0 Fe 0. 02 --O. 02 0 Al. 0. -0. 01 0 Ag--. 0. 05 0. 01 0. 03 Evaporation Loss, Percent 5 3. 8 Acid 1 Io 4. 59 2. 32 23. 30 18. 6 Viseoslty at 100 F. (05.) 36. 81 66. 42 8. 74 2. 60 28.86 160. 162 400 F. Evaporation Loss (Percent)- 8. 5 1. Ryder Gear Load, Lbs/Lu 3,000 3, 000+ 1 In these tests, 0.5 Wt. percent of phenothiazine was present as an oxidation inhibitor.

2 72 hour test at 347 F 3 24 hour test at 500 F.

propylene to the 0x0 process) in place of the C Oxo alcohol.

As shown by Table I, the triesters of the invention have good viscosity-temperature relationship, low pour points,

The products of A and B and a series of triesters of 75 low corrosion to lead and good oxidation and corrosion stability. Also, the load-carrying ability of these oils was very good, especially when compared to synthetic diester oils such as di(2-ethylhexyl sebacate which gives a Ryder gear load test of only about 2,000 pounds.

EXAMPLE II A. A monoester of the 1,2,4-butane tricarboxy acid was prepared in the general manner of Example I-A by esterifying 1 mole of C Oxo alcohol with 7 mole of the tricarboxy acid but using no catalyst. The resulting product was a honey colored, syrupy fluid.

1 wt. percent of the monoester product was added to a base composition consisting of di(2-ethylhexyl) sebacate and 0.5 wt. percent of phenothiazine as an oxidation inhibitor.

B. 0.75 wt. percent of the monoester was added to a base composition consisting of 40 volume percent of di(C 0x0) ladipate, 40 volume percent di(C Oxo) adipate, 20 volume percent di(2-ethylhexyl) seb acate, 1 wt. percent tricresyl phosphate and 0.7 wt. percent phenothiazine.

C. 0.75 wt. percent of the monoester was added to a complex ester of the general formula:

C 0x0 alcohol-(adipic acid-polyethylene glycol 200) adipic acid-C Oxo alcohol, where x averages 1.7, containing 0.5 wt. percent phenothiazine.

The composition prepared above were tested for lead corrosion, oxidation corrosion, and in the Ryder gear test, which test have been previously described. For comparison purposes, the base compositions per se, i.e. containing no monoester, were submitted to the same tests. The compositions tested and the results obtained are sum- As shown by the preceding table, the monoesters effectively raise the load-carrying ability of various diester and complex ester base oils while at the same time not having any significant adverse effect upon lead corrosion or the oxidation and corrosion stability of the base oil.

What is claimed is:

,1. A lubricating oil composition comprising a major amount or a synthetic ester lubricating oil prepared by complete esterification of a dicarboxylic acid and a minor load-carrying amount or a C to C alkyl partial ester of a C to C tnicarboxy aliphatic acid.

2. A lubricating oil composition according to claim 1, wherein said partial ester is a monoester and said loadcarrying amount is about 1 wt. percent.

3. A lubricating oil composition according to claim 1, wherein said synthetic ester prepared by esterification of dicarboxylic acid is a diester.

4. A lubricating oil composition according to claim 1 wherein said partial ester is a monoester of 1,2,4-butane tricarboxy acid and a C branched chain alcohol.

5. A method of lubricating rubbing parts which comprises applying to said parts a lubricating oil composition comprising a major proportion of a fully ester-ified synthetic carboxylic acid ester lubricating oil, and a minor, but load-carrying improving amount, or a C to C alkyl partial ester of tricarboxy aliphatic acid containing 6 to 7 carbon atoms.

6. A method according to claim 5, wherein said aliphatic acid is 1,2,4-buta-ne tricarb oxy acid.

7. A method according to claim 5, wherein said aliphatic acid is tricarbally-l-ic acid.

8. A method according to claim 5, wherein said alkyl :group contains 5 to '13 carbon atoms.

9. A method according to claim 5, wherein said parmarized in the following table: tial ester is a monoester.

TABLE II Lead Cor- Total 347 F. Oxidation-Corrosion Test Total Visc. at Ryder Example Lubricant 1 rosion Test Acid Acid 100 F. Gear,

(Mg/sq. Nuru- Num- (cs.) #/In 111.), 12 Hrs ber Cu Mg Fe A1 Ag ber Diester 1. 0 0. 38 04 01 0.0 02 0. 1. 3 12. 92 1, 910 II A Diester 1 1% Monoesten. 1. 0 3. 94 05 -i-. 04 04 04 04 3. 78 13. 15 2, 392 Diester 2 1, 650 II B Diestter 2 +.75% MODO- 2. 74 +.05 0. 0 +.02 01 02 2. 91 12. 77 2,167

es er. Complex ester 3 0.0 0.20 15 04 03 01 04 1. 63 37. 32 2, 730 II C Complex cster +0.75% 0.0 2.21 3,226

Monoester.

1 Di(2-ethylhexyl)sebacate +.5 wt. percent phenothiazine.

1 vol. percent di(C8Ox0) adipate, 40 vol. percent di(C1oOX0) adipate, 20 vol. percent di (Z-ethylhexyl) sebacate +1.0 wt. percent tricresyl phosphate-k0? wt. percent phenothiazine.

3 Complex ester of general formula: Ca 0x0 alcohol-(adipic acid-polyethylene glycol 200) x-adipic acid-Ca 0x0 alcohol, whcrcz averages 1.7+0.5 wt. percent phenthiazine.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Cohen et a1.: I. and E. Chem, vol. 45, No. 8, August Extreme Pressure Lubricants, Lubrication, vol. XLIII, No. 6 (June 1957), pub. by The Texas Company, 

1. A LUBRICATING OIL COMPOSITION COMPRISING A MAJOR AMOUNT OF SYNTHETIC ESTER LUBRICATING OIL PREPARED BY COMPLETE ESTERIFICATION OF A DICARBOXYLIC ACID AND A MINOR LOAD-CARRYING AMOUNT OF A C3 TO C20 ALKYL PARTIAL ESTER OF A C6 TO C7 TRICARBOXY ALIPHATIC ACID. 